Composite Higgs, Top Partners and the LHC

The models with a Higgs boson realized as a bound state of new strongly coupled dynamics and fea- turing the Goldstone symmetry protection mechanism, ensuring its lightness, represent a motivated scenario of New Physics at the TeV scale. We summarize the main ideas behind the formulation of these Composite Higgs (CH) models, focusing on the scenarios invoking the paradigm of Partial Compositeness, the mechanism of Standard Model fermion mass generation by mixing with compos- ite resonances. After reviewing the theoretical tools for the description of the CH setup, we derive structural phenomenological features, and in particular a general relation, valid for a broad class of models, between the Higgs mass and masses of the top partners, i.e. the composite resonances responsible for giving a mass to the top quark through Partial Compositeness. This relation implies that, for a “natural” theory, the top partners are restricted to have a mass below around 1.5 TeV, significantly below the other resonances, and they play a primary role for phenomenology. First of all, their direct observation at the Large Hadron Collider (LHC) seems unavoidable if the considered scenarios are indeed realized in Nature without accidental per-cent tuning. We develop a general minimally model-dependent framework, accounting for the Goldstone nature of the Higgs, in order to identify the best strategies for top partners searches at the LHC. Using current data we exclude a considerable part of the natural parameter space of the CH models. We present an estimate of the future LHC reach for two interesting channels. The light top partners could also manifest themselves in indirect way, contributing to the low-energy observables of the ElectroWeak Precision Tests. Our analysis shows that in this case top partners play an important role for compatibility of CH scenarios with experimental data, in particular we identify a new, potentially large, logarithmically divergent contribution to the S parameter. At the same time we show that the effects dependent on the details of the UV dynamics are important and can give contributions comparable with IR effects, and thus must be taken into account, as for instance the four-fermion operators contributing to Zbb coupling